The present invention relates generally to a fuel cell. More specifically, the invention relates to a fused carbonate fuel cell using methanol as a fuel.
A fuel cell is a power plant for directly converting chemical energy having a fuel into electric energy by an electrochemical reaction. It is noticeable that the fuel cell serves as an efficient and clean energy source since it does not burn the fuel. Such a fuel cell is defined as a cell which continuously removes a product by continuously supplying a reactant from the outside using the construction of its cell system as it is, and can be used as a permanent generating system if the construction of the cell system, the life of the fuel supply system and so forth permit.
In order to cause an electrochemical reaction in the fuel cell to generate electricity, a fuel gas, which is obtained by reforming a usual fossil fuel, such as a petroleum, a natural gas (methane) and a coal, into hydrogen (and carbon monoxide), is supplied to the cell to cause an electrode reaction in the cell. This fossil fuel reforming type fuel cell generating system uses a phosphoric acid aqueous solution, a fused alkali carbonate, a solid electrolyte or the like as an electrolyte. The second-generation system is a fused carbonate fuel cell which uses a fused carbonate. In this fused carbonate fuel cell, although the fuel gas can be generated from methane (CH.sub.4) ethane(CH.sub.3 CH.sub.3) or the like, methane obtained as a natural gas (or a substitute natural gas) is often reformed to be used as the fuel gas. In a case where methane is reformed to be used as the fuel gas, the reforming temperature of methane is in the range of from 750.degree. C. to 900.degree. C., and is higher than the operating temperature of the fused carbonate fuel cell which is in the range of 600.degree. C. to 750.degree. C.
An example of the aforementioned internal reforming type fuel cell, which uses a fuel obtained by reforming methane, is disclosed in Japanese Patent Laid-Open No. 6-310158. In a case where methane is used as a fuel of a fuel cell, it is required to carefully carry out the transportation, control and so forth of the fuel and to strictly store the fuel in a spherical tank or the like, since methane is basically a colorless, odorless and flammable gas. In order to eliminate such administrative problems of methane, although there is a method for liquefying a natural gas, there is also proposed a fuel cell using, as a fuel, an alcohol, which can be easily transported and controlled and which is an inexpensive liquid. The alcohols serving as a fuel include methanol (=methyl alcohol CH.sub.3 OH), ethanol (=ethyl alcohol C.sub.2 H.sub.5 OH), dimethyl ether (C.sub.2 H.sup.6 O) and propane (CH.sub.3 CH.sub.2 CH.sub.3). All the reforming temperatures of these alcohols are 450.degree. C. or higher. Among these alcohols, methanol, which is easily available as a main component of a liquefied natural gas (LNG) and controllable, is most preferred.
In a case where, e.g., methanol (CH.sub.3 OH) is reformed to be used as a fuel gas, the chemical reactions expressed by the following formulae (1), (2) and (3) occur substantially at the same time. EQU CH.sub.3 OH+H.sub.2 O=3H.sub.2 +CO.sub.2 (1) EQU H.sub.2 O+CO=H.sub.2 +CO.sub.2 (2) EQU 2CO=CO.sub.2 +C (3)
Formula (1) shows a methanol steam reforming reaction, formula (2) shows a water gas shift reaction, and formula (3) shows a Boudard reaction.
The aforementioned steam reforming occurs at a relatively low temperature of 200.degree. C. to 300.degree. C. A gas, which is obtained by vaporizing an alcohol such as methanol or ether and mixing the vaporized alcohol or ether with steam, may be used as a fuel. In a case where methanol is used as a fuel, a reforming reaction for converting methanol into water gas occurs, so that a reformer for allowing a reforming reaction is provided outside a fuel cell to supply the reformed fuel to the fuel cell. Such a reformed fuel is supplied to the fuel cell via a fuel gas pipe and a manifold, so that the path for supplying the fuel gas is relatively long.
In the aforementioned conventional fuel cell, after the reforming of methanol, the operating temperature of the fused carbonate fuel cell is in the range of from 600.degree. C. to 750.degree. C. On the other hand, in a case where a fuel reformed at a temperature of 200.degree. C. to 300.degree. C., such as methanol, is used, there is a problem in that carbon is deposited while supplying a fuel gas to the fuel cell as shown in the aforementioned formula (3). The deposition of carbon is a common problem in a case where alcohols having a reforming temperature below 450.degree. C. are used as a fuel gas.
At a temperature range of about 500.degree. C. to about 550.degree. C., the Boudard reaction represented by formula (3) is easy to occur in comparison with the shift reaction represented by formula (2), so that carbon is easily deposited. Since the deposited carbon is a solid, it is adhered to the fuel gas pipe, through which the fuel gas passes, so that the fuel gas pipe may clog up after the fuel cell is used for a long time. The fuel cell, which can not supply the fuel gas due to the clogging of the fuel gas pipe, can not generate electricity, so that the operating temperature during the generation of electrical energy must be temporally decreased to disassemble the gas pipe provided for the fuel cell, to remove carbon or change the gas pipe itself. This is a critical problem in fuel cells.
FIG. 1 shows the relationship between various chemical reactions occurring in a fuel cell and temperature. It can be clearly seen from this graph that a cell reaction occurs at a temperature above 650.degree. C., and the reforming temperature of methane (CH.sub.4), which allows methane to react with water to generate a hydrogen containing gas, is above 600.degree. C., at which carbon is not deposited. On the other hand, the reforming temperature of methanol (CH.sub.3 OH), which allows methanol to react with water to generate a hydrogen containing gas, is below 450.degree. C. Therefore, in the case of methanol, before the temperature reaches a temperature range for causing a cell reaction after reforming, it passes through a temperature range of 500.degree. C. to 550.degree. C. Since carbon is deposited by the Boudard reaction in this temperature range as mentioned above, if the reformed methanol is gradually heated to the temperature range of the cell reaction, carbon is adhered to the inner wall of the passage for supplying methanol.
Referring to FIG. 2, the aforementioned chemical reactions will be described in detail below.
FIG. 2 is a graph showing the relationship between the chemical reaction rate and the amount of deposited carbon versus temperature. In FIG. 2, the solid line shows the chemical reaction rate, and the broken line shows the amount of deposited carbon. In a temperature range above 550.degree. C., the reaction rate of the carbon deposition increases, whereas the amount of deposited carbon decreases as the temperature rises if the amount of humidification (the amount of steam contained in the fuel) is the same. On the other hand, in a temperature range below 500.degree. C., carbon is easily deposited, whereas the reaction rate decreases greatly in accordance with Arrhenius' equation.
Therefore, in a case where methanol used as a fuel gas is directly reformed outside the fuel cell, there is a problem in that carbon is deposited in a fuel cell (e.g., a fused carbonate fuel cell) having an operating temperature of 600.degree. C. to 700.degree. C.
Although it is also possible to inhibit carbon from being deposited by increasing the amount of humidification, the generating efficiency decreases as the amount of humidification increases. Therefore, in order to inhibit the deposition of carbon without decreasing the generating efficiency, a fuel gas pipe (including a manifold), which is provided between a tank for storing a fuel gas and a fuel cell body (usually called a laminated body) for receiving the fuel gas and through which the fuel gas having a temperature range of 500.degree. C. to 550.degree. C. passes, must be as short as possible.
However, since the conventional manifold is an external manifold provided outside a fuel cell body, it is naturally cooled in the installation environmental conditions of the fuel cell to a lower temperature than the operating temperature by about 100.degree. C. to about 150.degree. C. Therefore, the temperature of the manifold is in a temperature range, in which carbon is deposited, so that carbon is deposited.
In addition, unless the reforming conditions (temperature and so forth) are controlled immediately after methanol reaches the anode (electrode) of the fuel cell body, the reforming reaction occurs at the inlet of the anode, so that methanol is reformed to locally decrease the temperature to the range of from 500.degree. C. to 550.degree. C. If the temperature decreases, there is a problem in that carbon is deposited.
Moreover, since methanol has a low vaporization temperature of about 65.degree. C. and is easily reformed, it is not possible to surely grasp which portion of the fuel cell system allows vaporization and the reforming reaction to occur, so that it has a great influence on the redundancy and the generating efficiency of the fuel cell system.
In addition, if the reforming reaction and the endothermic property during vaporization can be utilized, it is possible to obtain a uniform temperature distribution of the fuel cell body and to use the reforming reaction and the endothermic as means for cooling a portion to be locally cooled. However, this method can not be easily carried out, and the reforming reaction is difficult to occur.